Method of electroplating stainless steels and irons



Nov. 7, 1950 C. B METHOD OF ELE ATCHELLER CTROPLATING STAINLESS S LS AND IRONS Oct. 11, 1944 INVENTOR.

J/e/mn/s fia/c/vaV/er ATTORNEY Patented Nov. 7, 1950 METHOD OF ELECTROPLATING STAINLESS STEELS AND IRONS Clements Batcheller, Glens- Falls, N. Y.

Application October 11, 1944, Serial No. 558,163

My invention relates to improvements in the art of electroplating stainless steels and irons.

All metals which are electroplated on a ferrous base under any of the present methods of plating offer little protection against the ultimate corrosion of such base due to the fact that the platings are crystalline in structure and vary in porosity, depending either on the metal itself or its method of application.

One of the precedent requirements which is now considered absolutely essential to the successful electroplating of any metal base is that the base be virtually chemically clean. That is to say, all foreign matter on the surface thereof, such as oxide films, scale, grease, etc., must be completely removed to expose the bare metal or alloy before an adherent metal plating can be applied thereto.

Doctor Mantell in his standard text book Industrial Electro-Chemistry, published in 1940, says:

Preparation of articles for plating is as important as the plating itself for the production of high quality finishes of adherent impervious metal coatings. It is of the utmost importance that any article to be plated be thoroughly clean. Foreign materials likely to adhere to the surfaces are of two kinds; (1) oxides or related products of corrosive influences, such as scale, tarnish or rust; and (2) organic substances such as grease, oil and various forms of dirt. In general, if the grease and corrosion products be removed, other foreign substances will also be eliminated. Generally the corrosion products are oxides removable either by mechanical abrasion or by pickling involving solution in acids.

Quite contrary to the long accepted teachings in the plating art I have discovered that it is possible not only to produce a high grade adherent electroplate directly upon a film of metal oxide which is substantially integral with a ferrous base, but that the presence of such oxide film between the base metal and the plating serves to an appreciable degree as a barrier against the corrosion of the ferrous base, and thus prevents the exudation of rust through the crystalline structure of the electro-deposited metal. The protective films which I apply to the stainless irons or steels are considered to be ferro-ferric oxides, although oxides of chromium, nickel and other alloying constituents in the base metal may be present but in a much lesser quantity than the oxides of iron.

I form the oxide film on the stainless steels and irons by treating the material in a hot solution of sulfuric acid, water and an etching inhibiting agent, preferably an etching inhibiting oxidizing agent. The quantity of sulfuric acid employed should be such that, when combined 2 Claims. (01. 204-29) with the water and, in the absence of the inhibiting agent, it will quickly attack and rapidly dlssolve a thin specimen of the steel to be filmed when immersed therein. The quantity of the etching-inhibiting agent which may then be added to the sulfuric acid-water solution should be at least sufiicient to prevent the etching of the surface of a specimen of the steel when immersed therein and when said solution is at the temperature to be used in the treatment, which is preferably comparatively high, say from 180 to 220 F. Temperatures lower than 180 may be employed, even room temperatures, but the process proceeds much more rapidly within the preferred temperature range stated above.

As an etching-inhibiting agent I prefer to use chromic acid and I prefer to form it in the sulfuric acid-water solution by incorporating a chromate or a dichromate forming salt therein such, for example, as sodium, potassium or ammonium dichromate. However, it is to be understood that chromic acid may be added as such to the sulfuric acid-water solution. Where chromic acid per se is employed the sulfuric acidwater solution is preferably activated by dissolving therein a small portion of scrap metal of the type to be treated before the chromic acid is added, or by adding small quantities of iron and chromium sulfates. I

The quantity of free sulfuric acid in the solution as finally formed, if the chromic acid is formed in the solution by adding a chromate or dichromate, is somewhat less than that in the initial sulfuric acid-water solution dueto the chromic acid forming reaction, and the final bath employed may contain the following ingredients in about percentages stated:

Ingredient: Percentage by weight Water to 55 Free sulfuric acid (1.84 sp. gr.) 15 to 55 Chromic acid 2 to 20 Iron sulfate .01 to 10 Chromium sulfate .01 to 10 Other sulfates or bisulfates and impurities Balance The preferred bath, however, for use in pretreating the steel or iron contains the following ingredients in about percentages stated:

Ingredient: Percentage by weight Water 40 to Free sulfuric acid (1.84 sp. gr.) 30 to 50 Chromic acid 6 to 10 Iron sulfate 0.1 to 1 Chromium sulfate 1 to 2 Other sulfates or bisulfates and impurities Balance By simply immersing the article to be plated in one of the above solutions, preferably with the upon the analysis of the alloy, the surface treatment to which it has been subjected, and the temperature of the solution, a film of dark colored oxides is formed substantially integral with the surface of the metal. Such a film contains oxides of the elements in the alloy, such as iron, chromium and nickel but the treatment does not in any way affect the surface texture and finish of the metal. That is to say, if the surface is highly polished the oxide film will be highly polished; and if the surface is matte, the oxide film will be matte. This film is extremely thin and its thickness is measurable only in Angstrom units.

The surface of the alloy iron or steel, to be plated may, if desired, be roughened, before it is oxidized, by sanding, pickling, graining, etching or otherwise to obtain the advantages which such treatment may afford in providing a mechanical bond between the base material and the plating although, in the case of stainless steel strip, for example, a very excellent bond is obtained where the;surface, before the oxidation treatment, has been merely subjected to the coarser mill finishes. After the film forming treatment the article should be washed to remove the acid and, before plating, especially where the oxidized surface is smooth, as in the case where the surface finish of the base metal before oxidation is very smooth, the article should be given a quick anodic dip in a solution of sodium hydroxide at ordinary room temperature and at a low current density, to insure that the surface of the oxide film is chemically clean. The anodic-treatment however must not be such, or so long continued, as to remove the oxide film. A plating of copper, zinc,

nickel, chromium, cadmium, gold, silver or any other metal susceptible to electroplating may then be applied thereto by any of the well known,

appropriate, electroplating processes for metal to be plated. However, in plating with copper an alkaline electrolyte should be employed because the commonly used copper sulfate-sulfuric acid solution attacks the oxide color film and also the metal beneath the film. Furthermore, the iron in the base material tends to throw the copper out of solution.

The drawing is a fragmentary cross section of a plated sheet which is drawn to a greatly exaggerated vertical scale.

In the drawing-- 1 represents the ferrous base while 2 and 2 indicate the oxide film on each side thereof, and 3 is'the outer layer of plating, here shown on only one side of the base.

While the surfaces of stainless steels are in-' herently highly resistant to corrosion, they are subject to rusting and pitting when exposed to attack by certain media. However, the corrosion resistance of steels and irons containing chromium in a quantity, say 7 or-8%, by weight, sufficient to withstand treatment in the sulfuric acidwater-inhibitor solution in. the manner which I have described, is very greatly enhanced by the presence of the dark colored oxide film formed thereon by said treatment, and bleeding or seepage of rust from the iron base through the plating overlying-said oxide film is substantially prevented.

.What I claim is:

,1. The method of plating the surface of a stainless steel alloy containing at least 7%, by weight, of chromium which comprises forming in and integral with said surface an adherent, dark colored film of oxide by merely immersingsaid steel in a solution containing, by weight- Percent Water to Free sulfuric acid (1.84 sp. gr.) 15 to 55 Chromic acid 2 to 20 Iron sulfate 0.01 to 10 Chromium sulfate 0.01 to 10 Minor quantities of other sulfates,

bisulfates and impurities Balance and thereafter electroplating said oxide filmed surface with copper in an alkaline electrolyte.

2. The method set forth in claim 1 in which the treating solution is maintained at a temperature between about F., and about 220 F.

CLEMENTS BATCHELLER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain Dec. 30, 1936 OTHER REFERENCES Journal of the ElectrodepositorsTechnical Society, vol. 14 (1937-38), pages 47, 48; vol. 13 (1937) Paper No. 13, pages 1, 2, 8.

Number Number 458,940 

1. THE METHOD OF PLATING THE SURFACE OF A STAINLESS STEEL ALLOY CONTAINING AT LEAST 7%, BY WEIGHT, OF CHROMIUM WHICH COMPRISES FORMING IN AND INTEGRAL WITH SAID SURFACE AN ADHERENT, DARK COLORED FILM OF OXIDE BY MERELY IMMERSING SAID STEEL IN A SOLUTION CONTAINING, BY WEIGHT- 